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Background and aims - A novel species of Navicula has been recently discovered in rivers of the central Appalachian Mountains. USA. The purpose of this paper is to formally establish this new species and to determine whether it had been overlooked in the past or is a recent arrival to the area.

Methods - We studied historical collections made in Pennsylvania, Virginia and adjacent areas since the 1940s and housed at the ANSP Diatom Herbarium. Light and scanning electron microscopy was employed to study the morphology of the new species and similar Navicula taxa, including those commonly reported at the same locations, or originally described from the rivers of the eastern United States, such as N. radiosafallax, initially established as N. radiosa var. parva by J.H. Wallace.

Key results - The new species has an unusual shape of the external raphe slit which is undulate toward the proximal ends. This character was so far found in a single other Navicula species known only from Europe. Examination of historical materials confirmed that N. eileeniae was absent from collections made in the eastern United States before 2007, but has become progressively more common and abundant in the last decade. The prevalence of this species in streams of low to moderate mineral and nutrient content suggests that its recent expansion cannot be explained by pollution. Climate change is also unlikely to have caused the northward expansion of N. eileeniae, as it has never been recorded south from central Appalachia.

Conclusions - The results of this study confirm that a population of previously unknown Navicula species has been expanding in Central Appalachia and suggest that rapid shifts of distributional ranges not readily explainable by environmental changes may occur in protists.